Waveguide Stress Birefringence

When optical signal propagates in fibers, its polarization varies randomly;so the insensitivity on polarization becomes important requirement in optical communication system. For optical waveguide components based on interference principle, because the propagation constants of TE and TM modes are different, the output performance of the component differs even for the same wavelength, resulting shift of central wavelength;that phenomenon is called polarization sensitivity, so it is necessary to study polarization insensitivity and polarization compensation. Birefringence is the base of studying polarization sensitivity of optical waveguide component;Birefringence comes from two sources, one arises from waveguide geometrical structure and the other from stress during waveguide manufacture. It is easy to investigate geometrical birefringence because of well-developed numerical simulation technology such as finite-difference-method (FD), finite-element-method (FEM), beam-propagation-method (BPM). People have lucubrated polarization performance of familiar optical waveguide, e.g. silica waveguide based on silicon, SOI waveguide;but researches on stress birefringence of these waveguides are limited. Stress induces anisotropy and nonuniform index distribution for optical waveguide. This paper deduces semi-vectorial wave equation in anisotropic medium and deduces difference format and steps of nonuniform finite-difference-method for solving semi-vectorial wave equation in detail.The paper investigates thoroughly stress birefringence in three kind optical waveguide, i.e. silica waveguide based on silicon, SOI waveguide and latest silicon nano-waveguide. Some difficulties cumber researches on stress birefringence;the first is modeling of stress calculating model. At the beginning people regard the model as an Ordinary Plain Stress model (OPS), take strain in the direction of waveguide for zero, which is some different with the real case. Recently a modified model called Normalized Plain Stress model (NPS) has been suggested, bringing more accurateresults. The paper discuss both models, compares their results;the results show that the results of birefringence calculating by two models are close, but there are some difference in results from OPS model considering the absolute value of index change. The paper also employs sub-model technology for the first time to reduce calculation complexity and save time with maintenance of accuracy. Stress birefringence calculated by FEM and analytical solution are very close, which conform the validation of analytical formula..Stress birefringence in silica waveguide based on silicon is about in the order of 10"4, core dimension has little influence on stress birefringence. Stress birefringence can be reduced by adjusting coefficient of thermo-expansion (CTE) of upper cladding to around the one of silicon substrate. The results show that stress birefringence is reduced to about 10"6;adjusting CTE of lower cladding can't reduce the stress birefringence. Another method of eliminating stress birefringence is carving stress relief grooves;results show that birefringence almost disappear when the depth of grooves reaches some distance in lower cladding, but width of grooves has little influence on stress birefringence. Influence of distance between waveguides on stress birefringence attenuates with distance increases and the simulation shows that when the distance between adjacent two waveguides is more than three times larger than the width of waveguide, coupler between adjacent waveguides can be ignored, stress distribution can be regarded the same with the one in only one waveguide. Stress birefringence in SOI optical waveguide is in the order of 10"3, much larger than in silica waveguide;that indicates large birefringence in SOI optical waveguide. Simulations show that geometrical birefringence in SOI waveguide is also in the order of 10-3, the result of which can be used to eliminating birefringence in SOI waveguide as a new approach. The paper discusses adjusting the depth of upper cladding to realize cancellation of stress birefringence and geometrical birefringence, so eliminating the total birefringence of SOI waveguide.Stress birefringence in silicon nano-waveguide is in the order of 10"4~10"3, but the geometrical birefringence can be in 10"1 order, so it is difficult to employ cancellation of stress birefringence and geometrical birefringence to eliminating total birefringencein this kind of waveguide. But it is convenient to realize tiny adjustment of birefringence in silicon nano-waveguide by stress approach...